1. Field of the Invention
The present invention relates to materials and processes to prepare polycrystal and monocrystal forms for use as solid electrolytes in fuel cells. In specific embodiments, a fuel cell having oxygen or air/solid O.sup.2- (oxide ion) conducting lanthanum fluoride (as a single crystal) hydrogen configuration produces about 1 volt of open circuit potential at moderately low temperatures. In other specific embodiments, a fuel cell having oxygen or air/specific substituted alkaline earth lanthanide fluorides (either as a single crystal or poly crystal) that conduct O.sup.2- /hydrogen configuration also produce electricity at moderately low temperatures compared to the present art.
2. Description of the Relevant Art
Fuel cells convert chemical energy to electrical energy directly, without having a Carnot-cycle efficiency limitation, through electrochemical oxidation-reduction reactions of fuels. Several types of fuel cells have been or are being investigated at the present time. These may generally be classified as shown in FIGS. 1A and 1B, as Table 1, depending upon the kinds of electrolyte used and the operation temperature.
The solid electrolyte fuel cell which can be considered as the third generation fuel cell technology, is essentially an oxygen-hydrogen (or H.sub.2 --CO mixture) fuel cell operated at high temperature (ca. 1000.degree. C.) with a solid ceramic oxide material used as the electrolyte. At present, yttrium- or calcium-stabilized zirconium oxides have been used as the electrolyte. The mechanism of ionic conduction is oxygen ion transport via O.sup.2- anion in the solid oxide crystal lattice.
References of interest regarding fuel cells include the following:
B. V. Tilak, R. S. Yeo, and S. Srinivasan (1981), "Electrochemical Energy Conversion-Principals", in "Comprehensive Treatise of Electrochemistry" Vol. 3: Electrochemical Energy Conversion and Storage (J. O'M. Bockris et al., editors), pp. 39-122, Plenum Press, New York.
K. K. Ushiba, (1984), "Fuel Cells", Chemtech, May, pp. 300-307.
A. McDougall (1976), "Fuel Cells", Energy Alternatives Series (C. A. McAuliffe, series editor), The Macmillan Press Ltd., London.
T. Takahashi (1984), "Fuel Cells (Japanese)", Chemistry One Point Series 8 (M. Taniguchi, editor), Kyoritsu-shuppan, Tokyo, Japan.
J. D. Canaday, et al. (1987), "A Polarization Model for Protonic Solid Electrolyte Fuel Cells," Int. J. Hydrogen Energy, Vol. 12, No. 3, pp. 151-157.
A. O. Isenberg (1981), "Energy Conversion via Solid Oxide Electrochemical Cells at High Temperatures," Solid State Ionics, Vols. 3/4, pp. 431-437.
Additional general information is found in "Fuel Cells" by E. J. Cairns et al. in Kirk-Othmer: Encyclopedia of Chemical Technology, (3rd Ed.), Vol. 3, pp. 545-568; and in "Fuel Cells" by O. J. Adlhart in Van Nostrand's Scientific Encyclopedia, 6th ed., D. M. Considine (ed), Van Nostrand Reinhold Co., New York, pp. 1296-1299, 1986, which are both incorporated herein by reference.
Lyall in U.S. Pat. No. 3,625,769 and Fouletier in U.S. Pat. No. 4,526,674 each disclose lithium/oxygen fuel cells.
Raleigh in U.S. Pat. No. 4,118,194 and Weininger in U.S. Pat. No. 3,565,692, each disclose halogen electrochemical cells or the like.
Solid electrolyte fuel cells have several advantages over the other types of fuel cells:
1. There are no liquids involved and, hence, the problems associated with pore flooding, maintenance of a stable three-phase interface, and corrosion are totally avoided.
2. Being a pure solid-state device, it poses virtually no maintenance problems. For example, the electrolyte composition is invariant and independent of the composition of the fuel and oxidant streams.
3. Inexpensive metallic oxides (ceramics) rather than expensive platinum can be used as the electrode catalysts.
4. The solid electrolyte fuels cells demand less feed gas preparation than the phosphoric acid cell (see FIG. 1), which requires a conversion of CO to H.sub.2 via the water-gas shift reaction, or the molten carbonate cell (see FIG. 1), which requires a carbon dioxide loop due to the use of carbonate ions for ionic transport.
The attraction of developing a solid electrolyte fuel cell is its simplicity. However, a high operation temperature (ca. 1000.degree. C.) is by far the most critical aspect of this type of fuel cell. Although high operation temperature produces high-quality exhaust heat that can generate additional electrical power, leading to a high overall system efficiency, maintaining the integrity of the cell components such as the interconnector is the most difficult challenge.
It is therefore desirable to develop alternative low temperature solid materials and composites for use as solid electrolytes in fuel cells that can be operated in a range of 25.degree.-600.degree. C. or lower (preferably about 200.degree.-400.degree. C., especially at ambient temperature). The present invention relates to the design of such low temperature solid electrolyte fuel cells based on non-oxide solid electrolytes, such as solid solutions of lanthanide fluorides (e.g. La.sub.1-x Sr.sub.x F.sub.3-x).
References of interest regarding such lanthanide fluorides include:
B. C. LaRoy et al., in the Journal of the Electrochemical Society: Electrochemical Science and Technology, Vol. 120, No. 12 pp. 1668-1673, published in December 1973, disclose some electrical properties of solid-state electrochemical oxygen sensors using vapor deposited thin films. Polycrystalline thin films of lanthanum fluoride solid electrolytes were investigated at ambient temperature.
In U.S. Pat. Nos. 3,698,955 and 3,719,564, Lilly discloses the use of rare earth fluorides such as lanthanum fluoride as solid electrolytes which are deposited as their films in a battery and a gas sensor respectively.
G. W. Mellors in European Patent Application No. 055,135 discloses a composition which can be used as a solid state electrolyte comprising at least 70 mole percent of cerium trifluoride and/or lanthanum trifluoride an alkaline earth metal compound, e.g. fluoride, and an alkali metal compound, e.g. lithium fluoride.
A. Sher, R. Solomon, K. Lee, and M. W. Muller (1967), "Fluorine Motion in LaF.sub.3 ", in "Lattice Defects and Their Interactions", R. R. Hasiguti, Editor, pp. 363-405, Gordon and Breach Science Publishers, New York.
A. Yamaguchi and T. Matsuo (1981), "Fabrication of Room Temperature Oxygen Sensor Using Solid Electrolyte LaF.sub.3 (Japanese)", Keisoku-Jidoseigyo-Gakkai Ronbunshu, Vol. 17(3), pp. 434-439.
M. A. Arnold and M. E. Meyerhoff (1984), "Ion-Selective Electrodes," Anal. Chem., Vol. 56, 20R-48R.
S. Kuwata, N. Miura, N. Yamazoe, and T. Seiyama (1984), "Potentiometric Oxygen Sensor with Fluoride Ion Conductors Operating at Lower-Temperatures (Japanese)", J. Chem. Soc. Japan, 1984(8), pp. 1232-1236, and "Response of A Solid-State Potentiometric Sensor Using LaF.sub.3 to A Small Amount of H.sub.2 or CO in Air at Lower Temperatures", Chemistry Letters, pp. 1295-1296, 1984.
M. Madou, S. Gaisford, and A. Sher (1986), "A Multifunctional Sensor for Humidity, Temperature, and Oxygen", Proc. of the 2nd International Meeting on Chemical Sensors, Bordeaux, France, pp. 376-379.
N. Yamazoe, N., J. Hisamoto, N. Miura, S. Kuwata (1986), "Solid State Oxygen Sensor Operative at Room Temperature", in Proc. of the 2nd Int. Meeting on Chemical Sensors, Bordeaux, France.
J. Meuldijk, J. and H. W. den Hartog (1983), "Charge Transport in Sr.sub.1-x La.sub.x F.sub.2+x solid solutions. An Ionic Thermocurrent Study", Physical Review B, 28(2), PP. 1036-1047.
H. W. den Hartog, K. F. Pen, and J. Meuldijk (1983), "Defect Structure and Charge Transport in Solid Solutions Ba.sub.1-x La.sub.x F.sub.2+x ", Physical Review B, 28(10), pp. 6031-6040.
J. Schoonman, J., G. Oversluizen, and K. E. D. Wapenaar (1980), "Solid Electrolyte Properties of LaF.sub.3 ", Solid State Ionics, Vol. 1, pp. 211-221.
A. F. Aalders, A. Polman, A. F. M. Arts and H. W. de Wijn (1983), "Fluorine Mobility in La.sub.1-x Ba.sub.x F.sub.3-x (0&lt;x&lt;0.1) Studied by Nuclear Magnetic Resonance", Solid State Ionics, Vol. 9 & 10, pp. 539-542.
A. K. Ivanovshits, N. I. Sorokin, P. P. Fedorov, and B. P. Sobolev (1983), "Conductivity of Sr.sub.1-x Ba.sub.x F.sub.3-x Solid Solutions with Compositions in the Range 0.03.ltoreq.x.ltoreq.0.40, "Sov. Phys. Solid State, 25(6), pp. 1007-1010.
H. Geiger, et al. (1985), "Ion Conductivity of Single Crystals of the Non-Stoichiometric Tysonite Phase La.sub.1-x Sr.sub.x F.sub.3-x (0.ltoreq.x.ltoreq.0.14)," Solid State Ionics, Vol. 15, pp. 155-158.
A. C. Lilly, et al. (1973), "Transport Properties of La.sub.3 F Thin Films," J. Electrochem Soc., Vol. 120, No. 12, pp. 1673-1973.
N. Miura, et al. (1987), "Development of Solid State Oxygen Sensor Operation at Room Temperature," Proc. 4th Int. Conf. Solid-State Sensors and Actuators, Toyko, Japan, pp. 681-684 (June).
Some of the structures described herein have been examined for usefulness as battery electrolytes. However, none of the references cited hereinabove, individually or collectively, disclose or suggest the present invention as described herein. All of the references cited above are incorporated herein by reference.